#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <string>
#include <vector>
#include <set>
#include <map>
#include <queue>
#include <stack>
#include <algorithm>

using namespace std;

int p[101], n, res;

int main(){
    scanf( "%d", &n );
    for( int i = 0; i < n; ++i ) scanf( "%d", &p[i] );
    for( int i = n-2; i >= 0; --i )
        if( p[i] >= p[i+1] ) {
            res += p[i]-p[i+1]+1;
            p[i] = p[i+1]-1;
        }
    printf( "%d\n", res );
	return 0;
}